Optical equipment including microscopes and specially designed magnifying devices have been widely used for inspection of small objects in a wide variety of industrial applications. In some cases they are used to permit inspection of small parts to see that they are free of scratches or other defects. In other cases optical magnification systems are used to verify certain critical dimensions of a manufactured part by magnification and comparison against a standard reticle or template. A common requirement for conventional microscope and optical comparator inspection systems is that there must be a direct, unobstructed optical line of sight to the object to be inspected, and more specifically to the particular surface of the object which is to be inspected. In the case of inspection of small manufactured parts, this ordinarily presents no problem as they can be positioned by hand or by suitable mechanical manipulators as desired within the field of view of the optical apparatus. However, if it is desired to inspect a surface or a part of an assembly, this can impose difficulties that rule out the use of conventional optical inspection techniques, if the part to be inspected is "buried" within the assembly, or if direct view of the part or surface in question is blocked by another part of the assembly.
An example is in the field of transducer head assemblies for flexible magnetic disk data storage systems. The so-called "floppy disk" media has recently found wide usage in a great variety of program and data storage applications for digital computer systems. The floppy disk media is a thin, generally planar flexible or pliant disk having magnetically sensitive surfaces and currently rotatable at speeds of approximately 360 rpm within a protective envelope or jacket cover. Data transfer to and from the floppy disk is achieved by one or more electromagnetic heads arranged within a transducer assembly. Both single sided and double sided floppy disk systems are currently in use. As the name implies, a single sided system makes use of a magnetic coating on only one side of the rotating flexible disk media with data transfer being accomplished by a single magnetic transducer brought into contact with the disk. In the case of a double sided floppy disk system, magnetic coatings on both sides of the flexible disk media are used covered with a magnetic coating, and a transducer assembly having a pair of magnetic heads is used for data transfer to both sides of the disk, with the heads positioned on either side of the disk, which is sandwiched between the heads. Double sided floppy disk systems offer advantages over single sided systems where data storage density and maximum data transfer rate considerations are important.
Transducers responsible for the electromagnetic transfer of data to and from the magnetic floppy disk generally comprise a plurality of magnetic core members interconnected and secured together to define the magnetic circuitry and having one or more coils. Generally the transducer structures used in association with data transfer to and from a floppy disk comprise a single channel read/write head, and may typically also include an erase head. The erase head generally comprises a plurality of erase cores for trimming a "track" of information written onto the floppy disk surface by the read/write head, and for erasing a pair of guard-band areas on each side of the trimmed track. Particular structural details and relative placement of the electromagnetic components which collectively comprise the various head portions of an electromagnetic transducer can vary from one design to another. However, all such head assemblies have a read/write head, and more particularly a read/write gap which must be positioned accurately with respect to the tracks of data on the disk.
In the case of a single-sided system, the transducer assembly is typically mounted upon a movable carriage structure which is radially indexed with respect to the floppy disk. Movement of the carriage structure enables the transducer carried thereby to access different desired circular "tracks" located at different radial positions on the magnetizable surface of the floppy disk. In the manufacture of a floppy disk data storage device, it is necessary that the read/write gap of the head be accurately possitioned within the carriage for compatibility with the defined data tracks on the disk, Positioning of the read/write gap can be specified in terms of three rectangular coordinate directions X, Y, and Z and an aximuth angle, with respect to defined reference positions and planes on the carriage.
In the prior art, optical techniques have been used for verification of the alignment, i.e. positioning, of the read/write gap for a single sided floppy disk head assembly. The transducer carriage is placed in a suitable holding in conjunction with a microscope-type viewing device which is aligned to look in a direction normal to the face of the transducer. The viewing device permits measurements of the actual position and azimuth of the read/write gap for comparison against a manufacturing tolerance.
Unfortunately however, optical verification techniques used successfully for verifying tansducer assemblies for single sided floppy disks cannot be used for double sided floppy disk head assemblies. In a double sided floppy disk system, a pair of transducers are provided, and they are mounted in a transducer assembly designed to position the heads for operative engagement of opposite surfaces or sides of the floppy disk, in generally opposed relationship to one another. The opposed transducers are positioned such that their respective head gaps are slightly radially offset relative to one another, in accordance with a specific industry standard, for minimizing magnetic flux interaction between the transducers.
In double sided floppy disk systems, the X, Y, Z, and azimuth positioning of the head gaps must be maintained within specified narrow tolerances with refernce to the carriage, or in the case of the "upper" core track centerline, with reference to the "lower" core track centerline. An optical inspection apparatus of the type used for verifying alignment of a single sided floppy disk head assembly cannot be used because there is no direct optical access or unobstructed direction of view for the faces of the transducers. Since the transducers face each other in operative position, each transducer blocks the view to the face of the other.
Various techniques are employed in industry for assembling and aligning the individual heads to the carriage, support arm, etc. at positions calculated to bring them into correct alignment after final assembly of the entire transducer head assembly. However, despite the care taken in alignment during the manufacturing process, there is still a need for direct verification of alignment of a completed unit in the final operative assembly according to the engineering specifications for the assembly.
The need for verifying alignment of the heads of a double sided floppy disk head assembly arises at several different times. At the completion of the manufacturing process for the head assembly, and prior to mating the head assembly to the disk drive, it is advantageous to verify head alignment in order to avoid difficulties later. This is particularly true in the case where the head assembly is built by a different company, or at a different facility, from the manufacture of the disk drive, as is the custom in the industry today. In such cases, the head manufacturer typically will test alignment of each head assembly prior to shipping, and the disk drive manufacturer will typically re-test the head assemblies on incoming acceptance testing before mating them with disk drives. Also, should trouble be encountered with the operation of a completed floppy disk data storage unit, there would be a need to once again check head alignment.
At present, all verification of double sided floppy disk head alignment is done by observing the electrical output of the heads while playing back from a standardized prerecorded disk, such as a "Dysan" or other reference diskette. A number of problems are inherent with this type of test. One problem is the quality and accuracy of the reference diskette itself. Since the Dysan diskette is made by writing the test tracks onto the diskette through the use of a head assembly, the accuracy of alignment of the heads used to make the Dysan diskette are critical, and there may not be an independent way of verifying that alignment.
A second problem area is the mounting of the head assembly carriage to a disk drive mechanism, which is necessary for running the test. Even if the heads were perfectly aligned to reference points on the head assembly carriage in accordance with an engineering specification, if there were errors in the mounting of the carriage to the disk drive, the heads could be misplaced with respect to the tracks on the reference diskette. The Dysan or reference diskette test is thus a test of the particular head assembly/disk drive combination, and not a test of the intrinsic alignment of the head assembly itself according to the written specification therefor.
A related problem is that because it is necessary to mount the head assembly to a disk drive before any measurement can be made, discrepancies in the disk drives used for separate tests of the same head assembly might fail to give repeatable results. This is particularly troublesome where the same head assembly is tested successively at the final quality control check at the head manufacturer's facility, and again at the incoming inspection at the disk drive manufacturer's facility, with different results for the two tests.
To solve these and other problems, the present invention provides apparatus that permits optical inspection and verification of alignment of the heads in a double sided floppy disk head assembly, that is not dependent upon characteristics of the disk drive mechanism. The optical verification system is not limited to a single specific head design, but is applicable to a wide variety of head assembly designs.